A chromatographic apparatus is a device for analyzing a sample to obtain a set of data representing a chromatogram which shows a signal intensity (e.g. output voltage) with respect to time (such a set of data are hereinafter called “chromatogram data”). A data processing system for a chromatograph processes the chromatogram data to detect each peak appearing on the chromatogram, identify a substance (component) corresponding to the detected peak by comparing the position (retention time) of that peak with a preset identification table, and calculate the concentration and/or amount of that substance (component) from the height or area of that peak. Such a data processing system (or method) is hereinafter called the “chromatographic data processing system (or method).”
Chromatographic data processing systems normally have some limits of the signal level that can be processed, due to hardware limitations on the signal-processing circuits including an A/D converter. For an input of a signal whose level is above the upper limit or below the lower limit, the system cannot perform correct calculations.
Apart from such a limit on the signal processing, it should also be noted that the reliability of a detection result obtained with a detector for a chromatographic apparatus varies depending on the signal level. For example, in a device used as a detector for a liquid chromatograph (e.g. an ultraviolet-visible spectrophotometer or a photo diode array detector), the non-linearity of the signal intensity normally becomes more noticeable as the component concentration in the sample increases, as shown in FIG. 9, which lowers the accuracy of quantitative determination. Meanwhile, the signal inevitably has various noises superposed on it. To deal with such a situation, it is preferable to dilute the sample before an analysis so that the concentration of each component in the sample will fall within a predetermined range (dynamic range).